N = int(input())

cache = dict()
cache[1] = 1
cache[2] = 2
def sugoroku(n):
    if n in cache:
        return cache[n]
    else:
        v = sugoroku(n-1) + sugoroku(n-2)
        cache[n] = v
        return v

if __name__ == '__main__':
    print(sugoroku(N))